Terahertz (THz) communication is widely considered as a key enabler for
future 6G wireless systems. However, THz links are subject to high propagation
losses and inter-symbol interference due to the frequency selectivity of the
channel. Massive multiple-input multiple-output (MIMO) along with orthogonal
frequency division multiplexing (OFDM) can be used to deal with these problems.
Nevertheless, when the propagation delay across the base station (BS) antenna
array exceeds the symbol period, the spatial response of the BS array varies
across the OFDM subcarriers. This phenomenon, known as beam squint, renders
narrowband combining approaches ineffective. Additionally, channel estimation
becomes challenging in the absence of combining gain during the training stage.
In this work, we address the channel estimation and hybrid combining problems
in wideband THz massive MIMO with uniform planar arrays. Specifically, we first
introduce a low-complexity beam squint mitigation scheme based on
true-time-delay. Next, we propose a novel variant of the popular orthogonal
matching pursuit (OMP) algorithm to accurately estimate the channel with low
training overhead. Our channel estimation and hybrid combining schemes are
analyzed both theoretically and numerically. Moreover, the proposed schemes are
extended to the multi-antenna user case. Simulation results are provided
showcasing the performance gains offered by our design compared to standard
narrowband combining and OMP-based channel estimation.

本文研究了采用低复杂度基于真实时间延迟的束变形消除方案和新颖的变体 OMP 算法来解决 THz 通信中基站天线阵列传播延迟超过符号周期时空响应变化和通道估计问题的宽带 massive MIMO 中混合组合问题，并与标准窄带组合和基于 OMP 的通道估计进行了实验比较。

宽带太赫兹大规模 MIMO 系统的信道估计和混合合并

Channel Estimation and Hybrid Combining for Wideband Terahertz Massive  MIMO Systems

With the increasing scale of antenna arrays in wideband millimeter-wave
(mmWave) communications, the physical propagation delays of electromagnetic
waves traveling across the whole array will become large and comparable to the
time-domain sample period, which is known as the spatial-wideband effect. In
this case, different subcarriers in an orthogonal frequency division
multiplexing (OFDM) system will "see" distinct angles of arrival (AoAs) for the
same path. This effect is known as beam squint, resulting from the
spatial-wideband effect, and makes the approaches based on the conventional
multiple-input multiple-output (MIMO) model, such as channel estimation and
precoding, inapplicable. After discussing the relationship between beam squint
and the spatial-wideband effect, we propose a channel estimation scheme for
frequency-division duplex (FDD) mmWave massive MIMO-OFDM systems with hybrid
analog/digital precoding, which takes the beam squint effect into
consideration. A super-resolution compressed sensing approach is developed to
extract the frequency-insensitive parameters of each uplink channel path, i.e.,
the AoA and the time delay, and the frequency-sensitive parameter, i.e., the
complex channel gain. With the help of the reciprocity of these
frequency-insensitive parameters in FDD systems, the downlink channel
estimation can be greatly simplified, where only limited pilots are needed to
obtain downlink complex gains and reconstruct downlink channels. Furthermore,
the uplink and downlink channel covariance matrices can be constructed from
these frequency-insensitive channel parameters rather than through a long-term
average, which enables the minimum mean-squared error (MMSE) channel estimation
to further enhance performance. Numerical results demonstrate the superiority
of the proposed scheme over the conventional methods in mmWave communications.

研究了毫米波通信中存在的空间宽带效应问题，提出了适用于毫米波大规模 MIMO-OFDM 系统的通道估计方案，可以从各个角度得到更高精度的频率参数和时域参数，进而通过下行链路进行更简易而准确的通道估计